Tester interconnect system

ABSTRACT

An automatic circuit tester (10) includes a scanner bus (50) that includes series of signal-path links (70A-H, 72A-H) and connectors (51) into which scanner boards (46, 47) are plugged. Each scanner board includes switches (74A-H, 76A-H, and 78A-H) that in different states can (i) connect together the links on opposite sides of the scanner board so as to forward signals along the link sequence and (ii) connect the link on one side to an instrument (18) or system pin (24) while isolating the link on the other side from the first link and the instrument or system pin.

BACKGROUND OF THE INVENTION

The present invention is directed to automatic circuit testers and in particular to an arrangement for interconnecting test instruments to the tester's system pins.

The high speeds of many modern electronic circuits have imposed stringent performance constraints on the automatic equipment for testing them. A specific constraint applies to the multiplexing of test instruments. Although complete testing of a particular circuit board may require that connections be made between test instruments and a large number of test points on the board, the number of test instruments required in the automatic test equipment typically is only a small fraction of the total number of possible board test points, since only a fraction of the test points are typically in use in any given part of the test. The test instruments can therefore be multiplexed.

But signal-speed considerations require that the multiplexing be carefully implemented. The typical way of distributing a common signal selectively to a large number of test points, or of according a large number of test points access to a common destination selectively, is to employ a bus arrangement, i.e., to run a common signal conductor among the various potential sources or destinations and tap onto the conductor at those various sources and destinations. Such an arrangement is advantageous because a common conductor provides most of the signal path to all of the large number of sources or destinations.

A drawback of such systems, however, is that they conventionally leave transmission-line "stubs." That is, signal paths typically extend from, say, a signal source not only to the current intended destination but also most of the way to destinations intended during other parts of the test. These alternate paths, or stubs, serve as sources of reflection, reducing signal fidelity. Test-signal fidelity is significantly degraded if the round-trip propagation time along a stub line approaches the signal rise time. A goal of the test-system designer concerned with higher-bandwidth signal fidelity is thus to minimize the cumulative stub-length effects so that the signal degradation is within the system rise-time specifications.

It turns out that this goal is usually achieved more readily for digital instruments than for analog instruments; since a much larger number of digital instruments (driver/sensors) must be provided in the test equipment, the switching matrices can usually be arranged, with little loss in flexibility, so that any given digital instrument is required to be multiplexed to only a small number of system pins. The various pins to which the digital instrument is to be multiplexed can therefore be disposed close together so that stub lengths tend to be relatively short.

The same is typically not true of analog instruments, of which there is typically a relatively small number, each of which must be multiplexed among a large number--and possibly all--of the system pins. For high-performance circuits with one-nanosecond rise times, cumulative stub lengths ten centimeters or more are by most measures unacceptable. If not all system pins can be placed within a ten-centimeter-wide region, therefore, it may not be possible to use a conventional bus organization.

One way to avoid this problem is to provide the switching at a location near to the common source or destination; the long-distance paths to the alternate sources (or destinations) are thereby isolated by the switching action and so do not act as stubs and cause ringing. But such an arrangement does not have the common-path advantage that characterizes bus systems, so it typically results in an inconveniently large number of long-distance signal paths.

SUMMARY OF THE INVENTION

The present invention is a circuit tester that retains the advantages of a bus-type organization without suffering the stub-length problems normally associated with conventional buses. According to the present invention, the tester includes connection modules that are connected to a signal bus for carrying signals between system pins and (typically, but not necessarily, analog) test instruments. Paths to system pins are provided through at least one of the connection modules, while a path to a test instrument is provided through at least one other connection module.

The signal bus differs from conventional buses in that each of a number of the bus lines that serve as signal paths between the DUT test points and the test instruments is repeatedly interrupted along its length to form a sequence of links. Each connection module is disposed between two adjacent links and separately connected to them. The connection module includes switching circuitry that can assume at least first, second, and third module states. In the first state, the connection module connects its two adjacent links together. In the second state, the module connects a first one of the links to a system pin or test instrument and leaves the second link isolated. In the third state, the first link is the one that is isolated and the second is the one connected to the system pin or test instrument.

To place a test instrument in communication with a system pin, the connection modules between those through which the test instrument and system pin respectively communicate assume their first states so as to provide continuity in the bus path between those two modules, which assume respective second and third states so as to connect the system pin and test instrument to opposite ends of the path thus created without connecting them to links beyond the path in either direction. This arrangement eliminates the stubs that would result from a conventional bus organization, but it affords the common-path advantages that such bus organizations provide.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features and advantages of the invention are described in connection with the accompanying drawings in which:

FIG. 1 is a block diagram of an automatic circuit tester that employs the teachings of the present invention;

FIG. 2 is a perspective view, partially broken away, of some of the elements of that tester;

FIGS. 3A and 3B together comprise a schematic diagram of a part of the tester's scanner circuitry;

FIG. 4 is an isometric view of one of the scanner boards, showing auxiliary boards mounted on it;

FIG. 5 is a schematic diagram of the interconnections among certain of the auxiliary boards on a main scanner board;

FIG. 6 is a schematic diagram of the interconnection of the relays included on auxiliary boards of the type depicted in FIG. 5;

FIG. 7 is a schematic diagram of the interconnections among others of the auxiliary boards on the scanner board of FIG. 4;

FIG. 8 is a schematic diagram of the interconnection of the relays on an auxiliary board of the type depicted in FIG. 7;

FIG. 9 is a schematic diagram of the connections to another of the auxiliary boards mounted on the scanner board of FIG. 4;

FIG. 10 is a schematic diagram of the interconnection of the relays on the auxiliary board of FIG. 9;

FIG. 11 is a schematic diagram of the interconnections among further auxiliary boards mounted on the scanner board of FIG. 4; and

FIG. 12 is a schematic diagram of the interconnection of the relays on auxiliary boards of the type depicted in FIG. 11.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

FIG. 1 represents in block-diagram form one of the many types of automatic circuit testers in which the teachings of the present invention can be employed. The tester 10 tests a device under test (DUT) 12 by using digital test instruments in the form of driver/sensors 14 to apply signals to the DUT and observe the resulting signals that appear on the DUT. In addition to the digital instruments 14, a tester may also use analog instruments, such as a waveform generator 16 or a digital voltmeter 18.

To connect the test instruments to the DUT 12, typical automatic test equipment employs a scanner 20 and a fixture 22. The scanner 20 provides a large number of fixed-position system pins 24, and these pins are used to carry signals to and from the DUT. However, they are not physically positioned to line up with test points on any particular circuit board, and signals on the system pins 24 have to be directed to different physical positions for every board type or family. This is the purpose of the fixture 22, which provides connections between the system pins 24 and fixture pins ("nails") 26 specifically positioned for the desired test points on the DUT 12.

For many DUTs, the number of necessary nails 26 is very large, but only a small number of them are employed at any one time. For instance, a DUT may have a large number of components, which in total require a large number of test points, but each test of an individual component or circuit may involve only the test points that electrically communicate with the particular terminals of that component or circuit and a few others, whose operation must be affected in order to isolate that component or circuit. In testing that particular component or circuit, the tester leaves all other test points idle. Subsequently, when the system tests other components or circuits on the board, it uses another small subset of the test points and thus another small subset of the nails 26.

Since each part of the test requires only a small subset of all of the nails 26, only a small subset of the system pins 24 are typically employed in any part of the test. In many cases it would therefore be wasteful to provide a separate test instrument dedicated to each system pin 24. This is particularly true of analog instruments, such as digital voltmeters 18 and waveform generators 16, since the number of such instruments used at a time is usually much smaller than that of the driver/sensors 14. The tester therefore includes the scanner 20, which is a matrix of switches and other circuitry that switches the connections between instruments and system pins 24 between bursts so that individual instruments can be used for different nails for different parts of a test.

The control circuitry for the tester may be embodied in a computer 28, a sequencer 30, and a scanner driver 34. To set the tester up for a burst, the computer 28 communicates with the scanner driver 34 by means of, for instance, industry-standard MXI and VXI buses 36 and 3 to specify the connections that the scanner 20 is to make between the instruments and the system pins 24. The scanner driver 34 responds by applying scanner-control signals to the scanner by means of a separate scanner bus to be described below. The VXI bus also serves as an instrument bus, carrying instrument-control signals such as those by which the computer 28 loads a pin memory 32 with values representing the signals that the individual test points are to receive or are expected to produce during the burst. The computer 28 may similarly program an analog instrument such as the digital voltmeter 18 if such an instrument is included as a standard part of the tester. Alternatively, an analog instrument such as the waveform generator 16, which may not be an ordinary part of the system and is not plugged into the VXI bus, may be connected to the scanner 20 and programmed separately, possibly by the computer 28.

For real-time control during the actual burst, the computer 28 turns control over to the high-speed sequencer 30, which clocks the driver/sensors 14 and the pin memory 32 and may control other instruments as well.

When the burst has been completed, the computer 28 reads the results from the pin memories 32 and, for instance, the digital voltmeter 18 and uses appropriate equipment such as a display 40 to produce an indication of the results, either then or after further bursts.

FIG. 2 shows the physical arrangement that certain of the elements of FIG. 1 might assume in an embodiment of the present invention. FIG. 2 shows that the VXI bus 38 is provided, in the conventional manner, as a backplane bus lying in a horizontal plane near the bottom of a tester chassis 42. The driver/sensors 14 and digital voltmeter 18 of FIG. 1 are provided by a number of circuit boards plugged into the VXI backplane 38 to receive the instrument-control signals that program and otherwise control their operation. FIG. 2 depicts only one of those boards 44, but a typical tester will employ many such boards arrayed physically in parallel. FIG. 2 omits the connectors into which such further boards would be plugged.

The circuit tester provides the scanner 20 in a plurality of circuit boards. Only two scanner boards 46 and 47 appear in FIG. 2, but the typical arrangement will employ a larger number. Also included in the scanner 20 of FIG. 1 is a scanner backplane bus 50, whose physical orientation is more or less parallel to that of the VXI bus 38. Scanner board 46 plugs into connectors 48 at the upper edge of board 44, where the board provides the ports at which the analog instrument or driver/sensors drive and/or sense signals. Scanner boards 46 and 47 also plug into connectors such as connector 51 on the scanner bus 50, from which they receive scanner-control signals. They also send and receive instrument and DUT signals over bus 50.

Although FIG. 2 shows edge connectors 48 as making the connection between boards 44 and 46, instrument boards not made by the tester manufacturer will not in general be arranged to mate in such a manner With the tester manufacturer's scanner boards. For this reason, the tester manufacturer preferably makes the height of its own instrument boards greater than the standard VXI-board height. This will leave a space between the scanner-board lower edge and the upper edges of the other suppliers' (standard-height) instrument boards, where, say, coaxial-cable connections can be made between the boards.

For further simplicity, FIG. 2 does not show the largely conventional connection of the VXI backplane 38 to the MXI bus by which the computer 28 communicates with the scanner 20. As was explained above, however, the computer does send signals to the VXI backplane 38, and some of those signals, appropriately decoded, are forwarded to the scanner 20 by the scanner driver 34, which is shown in FIG. 2 as being embodied in a circuit board oriented parallel to scanner boards 46 and 48. A "slot zero" board 52 makes the connections between the VXI backplane 38 and the scanner driver 34.

The upper edges of the scanner boards 46 and 47 provide connectors 54 and 56, which, together with corresponding connectors on other scanner boards not shown, contain part of the system pins 24 of FIG. 1. Also included on the upper edges of scanner boards 46 and 47 are connectors 58 for attaching coaxial cables from, for instance, the external waveform generator 16 of FIG. 1. (Of course, the waveform generator could be provided as the driver/sensors and the voltmeter are; i.e., it could be connected between a scanner board and the VXI bus.) It is through a discussion of the connection between the system pins and analog instruments such as the waveform generator 16 and the digital voltmeter 18 that the present invention will now be illustrated, although the broader principles of the invention can also be applied to connecting digital instruments.

FIGS. 3A and 3B depict in schematic form part of the switching functions performed by scanner board 46. The sixteen pairs of contacts 68 arrayed along the lower portions of FIGS. 3A and 3B represent contacts on the mating connectors 51 (FIG. 2) by which scanner board 46 is connected to eight pairs of conduction-path links in eight respective sequences of many links on the scanner backplane 50. (The scanner backplane 50 also carries scanner-control signals on lines not shown in FIGS. 3A and 3B; i.e., the scanner backplane 50 provides a control bus as well as the signal bus that consists of the link sequences) Each sequence forms a conduction path that runs the length of the scanner bus 50 when its component links are connected in series by switches on the various scanner boards, which, as will be explained presently act as connection modules for those link sequences. A first set of conductor links 70A-H (FIG. 3A) extends to connector 51 from the next connector to the left on the scanner backplane 50. A second set of links 72A-H (FIG. 3B) extends to connector 51 from the next connector to the right.

The purpose of these sequences of links is to carry signals between system pins 24 on one scanner board and analog instruments accessed through another scanner board. The manner in which this is accomplished involves the operation of switches 74A-H, 76A-H, and 78A-H, whose operation will now be described, as well as of certain other switches, whose operations will be described later.

Suppose that a waveform generator connected to a scanner board disposed to the left of the scanner board 46 depicted in FIGS. 3A and 3B has placed a signal on conductor link 70A in a manner to be described below. Suppose further that this signal is to be forwarded to one of the system pins 24 on the scanner board depicted in FIGS. 3A and 3B. This result is achieved by operating first-link and second-link switches 74A and 76A and internal switch 78A, all of which are associated with the link sequence of which link 70A is a part (the "A sequence"). Specifically, switches 74A and 78A are closed, while switch 76A remains open. Also open is the internal. switch 78E associated with the E sequence. The result of these switch states is that the signal from link 70A is steered to the upper circuitry in FIGS. 3A and 3B and is not forwarded to the next link 72A in the A sequence.

We now change the assumption so that the waveform-generator signal is coming from a source to the right rather than from one to the left but is still intended for one of the system pins 24 on the board represented by FIGS. 3A and 3B. To achieve this result, the states of internal switches 78A and 78E remain the same, but those of first- and second-link switches 74A and 76A are reversed. The switches thereby steer the signal from link 72A to the upper circuitry and isolate link 70A from the upper circuitry as well as from link 72A.

Now suppose that the signal originates in a waveform generator connected to a scanner board to the right of the board in FIGS. 3A and 3B and that its destination is a scanner board to the left of that in FIGS. 3A and 3B; that is, the signal is not intended to be directed to any of the system pins 24 on board 26 of FIGS. 3A and 3B. To achieve this result, internal switch 78A is opened, and the first-link and second-link switches 74A and 76A are both closed so as to connect first link 70A to second link 72A through the common node to which switches 70A and 72A are connected. That is, the board illustrated in FIGS. 3A and 3B acts to provide continuity in the otherwise interrupted A sequence, which includes links 70A and 72A.

Finally, suppose that a waveform-generator signal comes from the upper circuitry in FIGS. 3A and 3B in a manner that will be described later. Suppose further that this signal is to be forwarded to a system pin 24 on a scanner board to the left of that depicted in FIGS. 3A and 3B. Furthermore, assume that the signal is to be sent to the left along the A sequence of links, i.e., along link 70A. To achieve this result, internal switch 78E is open, internal switch 78A is closed, first-link switch 74A is closed, and second-link switch 76A is open.

The result of this transmission scheme is that it automatically adjusts the length of the signal path so as to minimize stub lengths. Specifically, when signals are to be transmitted from one scanner board to another, only the scanner-bus links disposed between those two boards are connected; the other scanner-bus links are isolated from those links, so the reflections that would otherwise result are avoided despite the common-path feature that results from a bus organization.

We now turn to the matrix organization depicted in the upper portions of FIGS. 3A and 3B. These matrices represent four parallel tree structures. Each tree structure has a respective one of four internal, "trunk" conductors 80A-D at its lowest level. Conductor 80A, the trunk of the first tree, is associated with the A and E sequences of links. Similarly, conductor 80B is associated with the B and F sequences, conductor 80C with the C and G sequences, and conductor 80D with the D and H sequences.

By means of tree switches 82-1A, 82-2A, 82-3A, and 82-4A, trunk conductor 80A at the lowest level of the A tree fans out to four "limb" conductors 84-1A, 84-2A, 84-3A, and 84-4A at the next higher level. Limb conductors 84-3A and 84-4A lead directly to coaxial-cable connectors 58, while connectors 84-1A and 84-2A both fan out further. By means of tree switches 86-1A, 86-2A, 86-3A, and 86-4A, for instance, limb conductor 84-1A fans out to four "branch" conductors 88-1A, 88-2A, 88-3A, and 88-4A. Each of these branch conductors, in turn, fans out to four respective system pins 24. By means of tree switches 90-1A, 90-2A, 90-3A, and 90-4A, for instance, conductor 88-1A fans out to four respective system pins 24-1, 24-2, 24-3, and 24-4.

By means of these and similar limbs and branches, trunk conductor 80A can be connected to any of the system pins 24 through a proper selection of switch closures. Since trunk conductor 80A can in turn be connected to left link 70A or 70E or to right link 72A or 72E, signals on the A and E sequences can be steered to any of the system pins 24. Inspection of FIGS. 3A and 3B reveals that each of the other trunk conductors 80B, 80C, and 80D can similarly be connected through a respective parallel tree to any of the system pins 24, so the system pins can additionally be connected to the link sequences associated with those trunk conductors.

The trunk conductors 80 can be connected selectively not only to the system pins but also to the analog instruments to which the scanner is connected. Switches such as switch 82-3A can connect a trunk conductor to a limb conductor such as conductor 84-3A, which leads to an analog instrument. If board 44 is an analog-instrument board, conductor 84-3A leads to the lower-edge contact pads that the instrument-board connector 48 (FIG. 2) receives. In order to show the range of features that a scanner board might have, however, FIG. 3A includes driver-sensor signal paths, which a scanner board of the type connected to a driver/sensor board would include. Such a scanner board would have its analog-instrument paths such as conductor 84-3A connected to the upper-edge connector 58, so FIG. 3A shows such a connection.

With a plurality of scanner boards plugged into it, the scanner bus can provide connections to as many as eight different instruments, although the system pins 24 on a single scanner board can be connected to only four of them at the same time. This means that, from the point of view of, say, system pin 24-17 (FIG. 3B), conductors 88-5A, 88-5B, 88-5C, and 88-5D represent the beginnings of paths that the tester provides to separate instruments on other boards when proper connections are made. Accordingly, by means of matrix switches 90-17A, 90-17B, 90-17C, and 90-17D, the DUT conductor path 96-17 associated with system pin 24-17 can be connected to any one of four instrument conduction paths that begin with respective branch conductors 88-5A, 88-5B, 88-5C, and 88-5D.

As was mentioned above, FIGS. 3A and 3B include the signal paths and other circuitry that would be provided by a scanner board of the type intended for connection to a driver/sensor board, which contains a group of digital instruments, i.e., driver/sensors. On such a scanner board, each of the system pins 24 is connectable not only to the analog instruments to which connector 58 and the scanner bus 50 provide connection but also to a respective dedicated driver/sensor on board 44. (Although the multiplexing approach illustrated here for analog instruments is also applicable to digital instruments, the illustrated embodiment does not multiplex the digital instruments, because it is intended for applications in which little hardware savings would result from doing so.) For instance, closure of a switch 102 connects system pin 24-1 to a conductor 100-1 that leads to a driver/sensor on board 44. Additionally, an overvoltage-protection circuit, provided on board 46 but not shown in the drawings, can be connected to system pin 24-1 by operation of a switch not shown in FIG. 3A but designated switch 104-1 in FIG. 6. Moreover, to isolate unused DUT test points and minimize stub lengths when these various options are employed, the board provides further switches 106-1 and 108-1.

For each of the thirty-two system pins 24, therefore, the scanner board 46 must provide eight switches: a driver/sensor switch such as switch 102-1, an overvoltage-protector switch such as switch 104-1 (FIG. 6), two isolation switches such as switches 106-1 and 108-1, and four matrix/tree switches such as switches 90-1A, 90-1B, 90-1C, and 90-1D. These are required in addition to the lower-level tree switches and the switches for selecting the mode of connection to the scanner bus 50.

The scanner provides this large number of switches as mechanical relays, which, because of their size, can present layout and stub-length problems. A structure of the type depicted in FIG. 4 reduces both the layout and the stub-length problems by extending the layout into a third dimension. The resulting arrangement simplifies the layout problem so as to enable the designer to provide more channels than an equivalent-sized two-dimensional layout would permit, and it reduces to a fraction the stub length required to provide the flexible layout illustrated in FIGS. 3A and 3B.

FIG. 4 depicts representative parts of scanner board 46, showing two connectors 110 and 112 by which it plugs into the driver/sensor board 44. FIG. 4 also depicts the connector 51 by which board 46 plugs into the scanner bus 50. The various layers of (multi-layer) board 46 provide many of the conduction paths depicted in FIGS. 3A and 3B.

However, the switches, which are all provided as mechanical relays, are mounted on auxiliary boards 114, which in turn are mounted on the main board 46 and extend transversely from its surface. There are fifty auxiliary boards, labeled 114-1 through 114-50, of which FIG. 4 shows only seven. Boards 114-1 through 114-46 contain eight relays apiece. Boards 114-47 through 114-50 contain six apiece.

Each board also contains circuitry, such as integrated circuits 116 and 118 on board 114-10, for controlling the relays on that auxiliary board. Circuitry provided on the main board 46 but not shown in the drawings receives instructions coupled to it by means of connector 51 from the scanner bus 50, which in turn receives those signals from the computer 28 by way of the scanner driver 34 (FIG. 2), the "slot zero" board 52 (FIG. 2), and buses 36 and 38 (FIG. 1). This circuitry receives signals from the computer 28 that designate the states that the relays are to assume, and it drives the relays so that they assume the proper states.

The first eight auxiliary boards 114 provide functions not germane to the present discussion and not represented in FIGS. 3A and 3B. FIG. 5 shows the interconnection of the next four boards 114-9, 114-10, 114-11, and 114-12, which contain the sixteen relays 90-1A through 90-1D, 90-2A through 90-2D, 90-3 through 90-3D, and 90-4A through 90-4D that interconnect system pins 24-1, 24-2, 24-3, and 24-4 with branch conductors 88-1A through 88-1D. As FIG. 5 shows, the main board contains branch conductive paths 88-1A, 88-1B, 88-1C, and 88-1D, to which corresponding terminals in each of auxiliary boards 114-9 through 114-12 are connected.

FIG. 6, which is a schematic diagram of the switching connections on board 114-9, depicts these terminals 120 on board 114-9 and shows that they lead to the matrix switches 90-1A, 90-1B, 90-1C, and 90-1D, which auxiliary board 114-9 provides. Another terminal 122 connects isolation relay 106-1 to system pin 24-1 by way of the main board, while yet another terminal 124 connects relay 102-1 to driver/sensor pin 100-1 by way of the main board. Further terminals 126 and 128 provide the connections between the auxiliary board 114-9 and the overvoltage-protection circuitry on the main board 46.

For the sake of clarity, FIG. 6 omits the relay-driver and other control connections needed to interpret and retain instructions regarding the required relay states. Raising these connections, as well as the connections illustrated in FIG. 6, out of the plane of the main board 46 greatly simplifies the conduction-path layout and, as will be explained later in greater detail, reduces the stub lengths required to achieve circuit versatility.

We now return to FIG. 5. Auxiliary boards 114-10, 114-11, and 114-12 are identical to board 114-9; each provides the switching necessary to connect its associated system pin 24 to one of the trees through their respective branch conductors 88-1A, 88-1B, 88-1C, and 88-1D, each of which fans out into four system pins. To provide connections for all thirty-two system pins, the circuitry of FIG. 5 is replicated eight times (4×8=32). The outputs of these groups of switches are eight four-conductor groups such as group 88-1 of FIG. 5, each of which contains four corresponding branch conductors, one for each of the four trees. FIG. 7 shows four of these four-conductor groups 88-1, 88-2, 88-3, and 88-4 leading to two auxiliary boards 114-17 and 114-18. The other four four-conductor groups lead in an identical manner to the thirty-fifth and thirty-sixth auxiliary boards 114, which are not shown in the drawings. These four boards contain the relays 86 that provide the concentration from the branch conductors 88 to the limb conductors 84.

FIG. 8 is a schematic diagram of the internal switch arrangement of auxiliary board 114-17. Terminals 130 of auxiliary board 14-17 receive the A-tree conductors from the four branch-conductor groups that lead to boards 114-17 and 114-18, while terminals 132 of that board receive the C-tree conductors from those groups. Corresponding terminals on board 114-18 receive the B-tree and D-tree conductors, respectively, from those groups. The identical pair of auxiliary boards that receives the other four branch-conductor groups segregates the A, B, C, and D conductors similarly.

Board 114-17 concentrates its eight branches into two limbs in a manner that is apparent from its schematic diagram in FIG. 8, which identifies its relays in accordance with the numbering scheme followed in FIGS. 3A and 3B.

FIG. 9 depicts auxiliary board 114-45, whose terminals 138 are connected to the limb conductor 84-1A from terminal 134 (FIG. 8) of auxiliary board 114-17, limb conductor 84-2A from the corresponding terminal on the thirty-fifth auxiliary board, and two lines 84-3A and 84-4A from two of the coaxial-cable connectors 58. Board 114-45 provides the concentration of all of the A-tree limb conductors into the A-tree trunk conductor 80A. Board 114-45 also receives all of the C-tree limb conductors at its terminals 140 and provides the concentration into the C-tree trunk conductor 80C. Board 114-45 is identical internally to the board 114-17 depicted in FIG. 8, but FIG. 10 depicts its internal connections schematically in order to show the correspondence of its relays to those depicted in FIGS. 3A and 3B.

The forty-sixth auxiliary board, not shown in the drawings, performs a similar concentration function for the B and D trees.

As FIGS. 3A and 3B show, each of the "trunk" conductors 80A-D can be connected selectively to two links in each of two respective link sequences. A separate auxiliary card for each tree provides the relays that perform this switching. FIGS. 11 and 12 depict one of those auxiliary boards 114-50, whose relays perform this switching for the A tree. Board 114-50 is internally identical to the forty-seventh, forty-eighth, and forty-ninth boards, which perform the same functions for the D, C, and B trees, respectively. As FIG. 11 shows, a common terminal 146 is connected to the trunk conductor 80A of the A tree. FIG. 12 shows the connections to the relays internal to board 114-50, showing that it contains relays 74A and E, 76A and E, and 78A and E, whose functions have been described above in connection with FIG. 3A. An auxiliary-board terminal 148 provides the connection from relay 74A to the conductor path on the main board that leads to the main-board terminal 68 by which the main board is connected to link 70A. In a similar manner, terminals 150, 152, and 154 provide the connections to links 72A, 70E, and 72E, respectively.

By returning to FIGS. 3A and 4, one can appreciate the particular advantages of the three-dimensional switch organization. As FIG. 3A shows, the sixteen relays 90-1A through 90-1D, 90-2A through 90-2D, 90-3A through 90-3D, and 90-4A through 90-4D form a cross-point matrix of switches that perform what can be thought of as a reciprocal treeing operation: not only can a branch 88-1 of a given tree be connected to any one of four system pins 24-1 through 24-4, but any of those system pins can also be connected to top branch 88-1 of any of the four trees.

Each of these selections results in transmission stubs; that is, although a main path is formed through the closed switch, a stub path branches from the main path to the open switches. It is important to keep these stub lengths short enough that the round-trip propagation time through them is short in comparison with the rise times of the signals that the system is intended to handle.

To understand the stub problem that the cross-point matrices present, suppose that the relays are to be set so that system pin 24-4 is to be connected into the A tree. This means that relay 90-4A will be closed, while relays 90-4B, 90-4C, and 90-4D will be open. A stub will accordingly be present on conductor 96-4, which, as FIG. 4 shows, is T-shaped. Specifically, the signal path that includes conductor 96-4 appears at the bottom of board 114-12 and forks when it reaches the top of that board. The signal is intended to proceed to the left to one side of relay 90-4A, which directs it to branch conductor 88-1A. However, the signal also propagates to the right until it reaches the open relay 90-4D, which reflects it. It is this distance, i.e., the distance from the fork to the open relay and back, that must be kept below a minimum dictated by the shortest signal rise time that the system is required to handle. Because the elongated relays are longitudinally disposed in parallel, conductor 96-4 can fan out to four relays without making this stub length excessive.

This same result could be achieved, of course, by arraying those relays on the main board 46 rather than on the auxiliary board 114-9. Because of the reciprocal nature of the treeing, however, stubs occur not only on the pin conductor 96-4 that spans corresponding branch conductors of the four trees but also on the branch conductor 88-1A that spans the four pin conductors, which are provided on different respective auxiliary boards 114-9 through 114-12.

Specifically, although the signal received from system pin 24-4 (FIG. 3A) is intended to propagate from the bottom of relay 90-4A on board 114-12 to the right along conductor 88-1A to board 114-17, it also propagates to the left along conductor 88-1A until it reaches open relay 90-1A on auxiliary board 114-9, from which it reflects back to the right. It is important for this reflection time to be short, and it therefore is also important for the stub length determined by the distance between the corresponding ends of relays 90-1A and 90-4A to be short. It would be virtually impossible to accomplish this goal and keep the stub on branch conductor 88-1A short, too, in a two-dimensional layout. In a three-dimensional layout, however, the stubs on both conductors can be kept short simultaneously by placing auxiliary boards 114-9 through 114-12 on their edges, physically in parallel, and relatively close together. A three-dimensional layout thus yields a higher packaging density and hence shorter signal stubs.

By employing the teachings of the present invention, therefore, one can obtain the versatility needed to avoid unnecessary investment in tester resources and at the same time achieve the high performance required for testing high-speed circuits. The present invention thus constitutes a significant advance in the art. 

We claim:
 1. In an automatic circuit tester comprising system pins adapted for connection to test points in a device under test and further including instruments for producing signals or monitoring signals applied thereto, the combination comprising:A) a signal bus comprising an ordered sequence of links, each of which has first, and second opposite ends; B) a plurality of connection modules, each connection module being associated with a respective pair of first and second adjacent links, connected to the second end of the first link of the pair associated therewith and to the first end of the second link of the pair associated therewith, each of at least three of the connection modules being associated with at least a respective one of a plurality of tester resources, at least one of the tester resources being a system pin and at least one other of the tester resources being one of the instruments, each connection module being operable among first, second, and third module states such that:i in the first module state thereof, each connection module couples the second and first ends of the first and second links, respectively, of the pair of links associated therewith to each other so as to provide continuity between those links but isolate them from each tester resource associated therewith; ii in the second module state thereof, each connection module connects the second end of the first link associated therewith to a tester resource associated therewith and isolates the first end of the second link associated therewith from the first link and each tester resource associated therewith; and iii in the third state thereof, each connection module couples the first end of the second link associated therewith to a tester resource associated therewith and isolates the second end of the first link associated therewith from each tester resource associated therewith and from the second link associated therewith; and C) a control circuit, arranged for selectively assuming one of a plurality of control states, each of which is associated with a different pair of connection modules and in which the control circuit operates one of the associated pair of connection modules to its second state, the other to its third state, and all connection modules between the modules associated with that control state to their first states, whereby, in each control state, the tester provides a signal path between tester resources associated with the connection modules associated with that control state while isolating those resources from links in the bus beyond the part thereof providing the connection between the connection modules associated with that control state.
 2. A combination as defined in claim 1 wherein:A) the signal bus comprises a plurality of ordered sequences of links, each of which has first and second opposite ends and corresponds to a link in each other sequence of links; B) each connection module associated with a pair of first and second adjacent links in the first-mentioned sequence of links is also associated with the links, denominated first and second links, respectively, corresponding thereto in each other ordered sequence of links and is connected to the second end of the first link of each pair associated therewith and to the first end of the second link of each pair associated therewith; and C) each connection module is operable among first, second and third module states associated with each ordered sequence of links such that:i in the first module state thereof associated with a given sequence of links, each connection module couples the second and first ends of the first and second links, respectively, of the pair of links associated therewith in the given sequence of links to each other so as to provide continuity between those links but isolate them from each tester resource associated with that connection module; ii in the second module state thereof associated with a given sequence of links, each connection module connects the second end of the first link associated therewith to a tester resource associated with that connection module and isolates the first end of the second link associated therewith in the given sequence of links from the first link in the same sequence and from each tester resource associated with that connection module; and iii in the third state thereof associated with a given sequence of links, each connection module couples the first end of each second link associated therewith in the given sequence of links to a tester resource associated with that connection module and isolates the second end of each first link associated therewith in the given sequence of links from each tester resource associated with that connection module and from the second link associated therewith on the same ordered sequence of links.
 3. A combination as defined by claim 2 comprising:A) a generally planar scanner back plane providing the signal bus; and B) a plurality of generally planar circuit boards connected to the scanner back plane so as to extend transversely thereof, each circuit board providing a different one of the connection modules.
 4. A combination as defined as claim 3 wherein each module includes:A) a common node associated with each sequence of links; B) an internal node associated with each sequence of links, each internal node being connectable by the module to a tester resource associated therewith; C) a first-link switch associated with each first link associated with that module and connected between the associated first link and the common node associated with the sequence of which the associated first link is a part, each first-link switch being closed in the first and second states of that module associated with the sequence of links to which the first link with which that first-link switch is associated belongs and being open in the third state thereof associated with that same sequence of links; D) a second-link switch associated with each second link with which that module is associated and connected between the associated second link and the common node associated with the sequence of links of which the associated second link is a part, the second-link switch being closed in the first and third module states of that module associated with the sequence of links to which the second link with which that second-link switch is associated belongs and being open in the second module state thereof associated with that same sequence of links; and E) an internal switch associated with each sequence of links and connected between the common node associated with that sequence of links and the internal node associated with that sequence of links, each internal switch being open in the first module state of that module associated with the sequence of links with which that internal switch is associated and being closed in the second and third module states thereof associated with that same sequence of links.
 5. A combination as defined in claim 1 comprising:A) a generally planar scanner back plane providing the signal bus; and B) a plurality of generally planar circuit boards connected to the scanner back plane so as to extend transversely thereof, each circuit board providing a different one of the connection modules.
 6. A combination as defined in claim 5 wherein each module includes:A) a common node; B) an internal node connectable by that module to a tester resource associated therewith; C) a first-link switch associated with each first link associated with that module and connected between the associated first link and the common node, the first-link switch being closed in the first and second states of that module and being open in the third state thereof; D) a second-link switch associated with the second link with which that module is associated and connected between the associated second link and the common node, the second-link switch being closed in the first and third module states of that module and being open in the second module state thereof; and E) an internal switch connected between the common node and the internal node, the internal switch being open in the first module state of that module and being closed in the second and third module states thereof.
 7. A combination as defined in claim 1 wherein each module includes:A) a common node; B) an internal node connectable by that module to a tester resource associated therewith; C) a first-link switch associated with the first link associated with that module and connected between the associated first link and the common node, the first-link switch being closed in the first and second states of that module and being open in the third state thereof; D) a second-link switch associated with the second link with which that module is associated and connected between the associated second link and the common node, the second-link switch being closed in the first and third module states of that module and being open in the second module state thereof; and E) an internal switch connected between the common node and the internal node, the internal switch being open in the first module state of that module and being closed in the second and third module states thereof. 